1. Field of the Invention
The invention relates to a semiconductor apparatus and specifically relates to a semiconductor apparatus, a method for growing a nitride semiconductor and a method for producing a semiconductor apparatus which are suitable for a light emitting device and a light receiving device such as a light emitting diode (LED), a laser diode (LD), a solar cell and a photosensor, and an electronic device such as a transistor and a power device.
2. Description of the Related Art
A nitride semiconductor containing Ga as a main constituent (a GaN semiconductor) is utilized as a material for an optical device such as a light emitting diode of blue light or violet light, a laser diode and a photodetector. Moreover, attention is given to the GaN semiconductor as a high-performance material for an electronic device as well, because the GaN semiconductor is capable of satisfying high frequency and high electric power, and is highly reliable.
Further, a light emitting diode using the GaN semiconductor is known (refer to Japanese Unexamined Patent Publication JP-A 4-321280 (1992), for example). An example of a structure of such a light emitting diode is shown in FIG. 17. A GaN buffer layer 21 is formed on a sapphire (Al2O3) substrate 20, and on the GaN buffer layer 21 is formed a growth layer made of a GaN semiconductor layer having a multilayer structure made by sequentially laminating an n-GaN layer 22 of the n-type semiconductor layer, an n-AlGaN cladding layer 23 of the n-type semiconductor layer, an InGaN light emitting layer 24, a p-AlGaN cladding layer 25 of the p-type semiconductor layer and a p-GaN layer 26 of the p-type semiconductor layer. In part of the growth layer, a region from the p-GaN layer 26 to (an upper portion of the n-GaN layer 22) is etched and removed, and thus part of the n-GaN layer 22 is exposed. The n-type electrode 28 is formed on an upper surface of the exposed region, and the p-type electrode 27 is formed on an upper surface of the p-GaN layer 26 of an uppermost layer.
Since production of a single crystal substrate of the GaN semiconductor is difficult, there is a need to form a semiconductor apparatus using the GaN semiconductor on a substrate made of a different material. Although sapphire is generally used as the substrate 20, a Si substrate, a ZnO substrate, an MgO.Al2O3 (spinel) substrate, a SiC substrate, a GaAs substrate and the like are tested other than the sapphire substrate.
In the case of growing the GaN semiconductor layers on the sapphire substrate 20, a problem is a lattice mismatch between them. A relation of lattice constants thereof is as shown below. GaN grows in a direction rotated through 30° from an a axis on a c plane of the sapphire substrate. Regarding sapphire, a lattice constant a (which is described as a1 in Table 1 described later) is 4.7580 Å. An interval value (which is described as a2 in Table 1 described later when a lattice rotates through 30° is 2.747 obtained by 4.758×1/1.732 (a numerical value obtained by multiplying a length of the a axis in a unit lattice of sapphire by 1/1.732 becomes a reference). On the other hand, regarding GaN, a lattice constant a is 3.1860 Å.
A ratio of the lattice mismatch of GaN with reference to sapphire becomes +15.98% (=100×(3.1860−2.747)/2.747). Thus, the lattice constant of sapphire is significantly different from the lattice constant of GaN. Consequently, a good quality crystal cannot be obtained when GaN is directly grown on sapphire. Besides, it is possible to consider a substrate made of a material of another kind in the same manner.
In a prior art, in order to increase crystalline quality of the growth layer, a buffer layer made of an AlN or GaN material which is amorphous or polycrystalline is formed on the (0001) plane of the sapphire substrate 20 in advance, and the GaN growth layer is formed on the buffer layer. The buffer layer has a function of reducing the lattice mismatch between the GaN growth layer and the sapphire substrate and increasing crystalline quality.
Further, in the case of a semiconductor apparatus such as a laser diode or a transistor which requires a better quality crystal, after the GaN semiconductor layer is once grown on the single crystal substrate 20, the single crystal substrate 20 is eliminated, and then the semiconductor apparatus is formed. This is because when the semiconductor apparatus is formed on a substrate 20 made of a different material from the semiconductor apparatus, a crystal defect which results from a difference in coefficient of thermal expansion occurs in a cooling process after crystal growth at high temperatures of 1000° C. or more.
Furthermore, in another prior art, there is also a method for growing the GaN semiconductor layer 22 in which a mask made by patterning a SiO2 thin film is formed and the GaN semiconductor is grown on the mask in a lateral direction, in order to avoid an influence by the lattice mismatch with the substrate 20.
However, since the ratio of the lattice mismatches between the sapphire substrate 20 and the GaN layer 22 is as large as 15.98%, the GaN growth layer 22 contains dislocations whose density is 108 to 1011 cm−2 even when formed via the buffer layer 21 made of an AlN or GaN material. Moreover, even a layer of a GaN crystal laterally grown after elimination of the sapphire substrate 20 contains dislocations whose density is 104 to 107 cm−2. They contain extremely large dislocations as compared with GaAs grown on a GaAs substrate containing dislocations whose density is 102 cm−2 to 107 cm−2.
The dislocation density of the GaN growth layer significantly restricts performance of a semiconductor apparatus to be produced from this, and moreover, there is a need to increase the amount of additive elements in the semiconductor layer for generation of sufficient carrier. This has a problem of deteriorating a characteristic of a semiconductor apparatus, such as a life, a withstand voltage, a driving voltage, consumed electric power (operation efficiency), an operation speed or a leak current.
Then, in a proposed art, growth of a nitride semiconductor on a diboride single crystal substrate expressed by a chemical formula XB2, in which X contains at least one of Ti and Zr, is proposed.
TABLE 1Coefficient ofLattice constantthermal expansion[Å][×10−6/K]ZrB23.16965.9TiB23.0303Sapphire(Al2O3)a1 = 4.7580, a2 = 2.7477.5GaN3.18605.6AlN3.11144.2BN2.5502InN3.545.7SiC3.084.3GaAs4.008.7Si3.842.6
Since the nitride semiconductor is formed with a good lattice match relation with the diboride single crystal substrate in this manner, a lattice defect in the growth layer is small, and crystalline quality of the nitride film is extremely good.
However, in the aforementioned prior art, for example, when GaN is crystal-grown as the nitride semiconductor on the diboride single crystal substrate, because of a change of a growth temperature in a growth process, B in the substrate diffuses into the crystal-grown GaN crystal, and a nitride semiconductor GaBN which contains a ternary 13 group (a former IIIB group element) is generated on an interface between the GaN and the substrate. In the case of BN, as shown in Table 1, a mismatch of lattice constants becomes as large as approximately 20%, as compared with GaN (for example, (2.5502−3.1696)/3.1696=19.5%). Therefore, in the case of GaBN, which is a ternary nitride semiconductor, a difference of lattice constants becomes significantly large as a mixed crystal ratio of B becomes large, unlike AlGaN, which is a ternary nitride semiconductor which the mismatch ratio of lattice constants is 2% or less. Consequently, a lattice defect occurs on the interface even when GaN is grown on the diboride single crystal substrate as described above, and a good quality crystal cannot be obtained.
In recent years, research and development of a nitride semiconductor which contains at least one selected from among B, Al, Ga, In and Ti have become active, and applied technique has rapidly developed. Then, at present, a light emitting diode of green, blue and ultraviolet, a laser diode of blue and violet, and the like using the nitride semiconductor is put to practical use.
In specific, (InN)x(GaN)1−x whose band gap covers from red to violet and (AlN)x(GaN)1−xN whose band gap covers from violet to ultraviolet are positioned as main materials among III group nitride semiconductors, because the former makes it possible to realize a device emitting light of blue green, blue, violet or the like which was not realized before, and the latter makes it possible to expect application as a light source for measurement, sterilization or excitation.
The III group nitride semiconductor is grown from vapor phase on a single crystal substrate made of sapphire, SiC, GaAs, Si or the like by a MOVPE (Metalorganic Vapor Phase Epitaxy) method.
The III group nitride semiconductor is a hexagonal symmetry, and a-axis lattice constants of InN, GaN and AlN are 0.311 nm, 0.319 nm and 0.354 nm, respectively. Moreover, respective lattice constants of (InN)x(GaN)1−x and (AlN)x(GaN)1−x are values in a middle range of the respective aforementioned lattice constants of InN, GaN and AlN depending on x.
However, since intervals between atoms that sapphire, SiC, GaAs and Si should achieve lattice matches with the III group nitride semiconductor are 0.275 nm, 0.308 nm, 0.400 nm and 0.384 nm, respectively (refer to Table 1), and substrates which completely achieve lattice matches are not obtained.
In the meantime, in another prior art a technique on a low-temperature buffer layer is proposed (refer to Japanese Examined Patent Publication JP-B2 4-15200 (1992) and Japanese Patent 3026087, for example).
It is possible to grow a good quality crystal on the lattice mismatching substrates described above by using these technique, however, penetration dislocations of approximately 108 cm−2 to 1011 cm−2 still exitss. Moreover, differences in the coefficient of thermal expansion between the above single crystal substrates and the nitride semiconductor are large, and a difference in contraction amounts after crystal growth at a high temperature of approximately 1000° C. causes cracks.
As a method for solving these problems, in another prior art a technique of growing a nitride semiconductor on the (0001) plane of a ZrB2 single crystal substrate is proposed (refer to Japanese Unexamined Patent Publication JP-A 2002-43223).
The ZrB2 single crystal substrate is a hexagonal symmetry, and a lattice constant of an a axis is 0.317 nm (refer to table 1), which completely achieves a lattice match with 0.26 of x of (AlN)x(GaN)1−x. Moreover, the coefficient of thermal expansion is 5.9×10−6 K−1, which is a close value to 5.6×10−6 K−1 of that of GaN.
Further, the ZrB2 single crystal substrate, whose resistivity is as small as 4.6 μΩ·cm, is electrically conductive. On the other hand, a sapphire substrate 110 generally used as a substrate up to now is insulative, and therefore, a light emitting diode formed on the sapphire substrate has a structure such that two electrodes 101 and 109 are disposed on the same plane side (the upper side of the substrate 110 in FIG. 18) as shown in FIG. 18.
The structure shown in FIG. 18 is a structure in which a low-temperature buffer layer 107, the n-type contact layer 106, the n-type cladding layer 105, a light emitting layer 104, the p-type cladding layer 103 and the p-type contact layer 102 are sequentially laminated on a sapphire substrate 110, and furthermore, a p electrode 101 is formed thereon. Moreover, an n electrode 109 is formed on an exposed surface of the n-type contact layer 106.
As described above, in the last one or two years, research and development of the technique of growing the nitride semiconductor on the ZrB2 single crystal substrate have progressed.
According to “Abstr. 13th Int. Conf. Crystal Growth, August 2001, 02a-SB2-20”, a technique of enabling growth of GaN on the (0001) plane of the ZrB2 single crystal substrate by an MBE method is proposed.
However, this technique has a problem that it is inferior in mass production because it uses the MBE method.
Further, a technique of growing GaN on the (0001) plane of the ZrB2 single crystal substrate by using an AlN buffer layer by the MOVPE method is also proposed (refer to Ext. Abstr. (62nd Autumn Meet. 2001); Japan Society of Applied Physics, 12p-R-14).
Further, in the prior art, a GaN film grown on the (0001) plane of a ZrB2 single crystal substrate by an MOVPE method has a problem that a surface shape thereof tends to become uneven as shown in FIG. 13.
As described above, in the case of growing a nitride semiconductor on the (0001) plane of the ZrB2 single crystal substrate by using an AlN buffer layer, it is desired to reduce unevenness appearing on a surface shape thereof.
However, according to both the techniques, in the GaN film grown on the (0001) plane of the ZrB2 single crystal substrate, a rocking curve FWHM (TILT) of (0002) plane omega scan by an X-ray diffraction method, which becomes an indicator of evaluation of quality, is approximately 1000 arcseconds, which is not sufficiently good (refer to “Study on the crystal growth and properties of group-III nitride semiconductors on ZrB2 substrate by metalorganic vapor phase epitaxy” master's thesis written by Yohei Yukawa, graduate school of Meijo University, 2001).
Furthermore, since AlN is insulative, when the light emitting diode or the like with the structure as shown in FIG. 8 as described later is produced, resistance from the nitride semiconductor layer to the substrate becomes high, and an operation voltage becomes high.
Still further, a band gap of AlN is as large as 6.2 eV, and therefore, it is difficult to decrease resistance by doping.
In the aforementioned prior art, when the nitride semiconductor is grown on the (0001) plane of the ZrB2 single crystal substrate, resistance from the nitride semiconductor layer to the substrate becomes high, and the operation voltage becomes high.
In recent years, a nitride semiconductor such as gallium nitride (GaN), indium nitride (InN) or aluminum nitride (AlN) is used as a material for an optical device such as a light emitting diode of blue light or violet light, a laser diode or a photodetector, because the nitride semiconductor is a compound semiconductor of direct transition type, and has a wide band gap.
Further, since the nitride semiconductor is capable of satisfying high frequency or high electric power, and is highly reliable, the nitride semiconductor is noted as a high-performance material for an electronic device.
Up to now, there is no substrate that achieves a lattice match with the nitride semiconductor. In a conventional art, the nitride semiconductor is grown by the use of a substrate made of a material, such as a sapphire substrate different kind from nitride semiconductor.
However, for example, regarding the sapphire substrate and GaN, a ratio of lattice mismatch is 13.8% and a difference in the coefficient of thermal expansion is 3.2×10−6/K, and there is a problem resulting from the mismatch such that dislocations of 108 to 1010 cm−2 arises in the GaN film because of a crystal defect caused on an interface between the sapphire substrate and the GaN film.
Further, because of the defect and thermal distortion, the GaN film is warped, and crystalline quality is significantly deteriorated.
Furthermore, considering production of a device such as a laser diode, the nitride semiconductor is formed on a substrate made of a material of different kind from the nitride semiconductor such as GaN and the like, and therefore, there arises such a problem that, in the case of forming a reflection surface of a laser resonator, cleaved planes of the substrate made of a material and the nitride semiconductor are different, and that formation by cleavage is difficult. Accordingly, a good quality nitride semiconductor substrate has been expected.
However, regarding the nitride semiconductor such as GaN, a melting point is high and a dissociation pressure of nitride is high at the melting point, and therefore, production of a bulk single crystal is difficult. Consequently, as described before, for example, by growing a thick film of GaN on the sapphire substrate and then separating the sapphire substrate, which is made of a material of different kind from GaN, and the GaN thick film, the nitride semiconductor apparatus is produced.
However, in the step of separating the substrate and the nitride semiconductor in the aforementioned production method, a method of abrading the substrate arises a problem that stress from the nitride semiconductor thick film becomes large as the substrate becomes thin, and that the stress acts on the substrate, thereby worsening a warp thereof and causing cracks.
In the meantime, as prior art another separation method, there is, for example, a method of separating by locally irradiating the interface between the sapphire substrate and the GaN thick film with a laser light beam and subliming the interface (a laser lift off method).
However, according to this method, only a small part of the interface is separated because an area irradiated is small, and stress concentrates on a small attaching part, with the result that cracks are caused. Moreover, since the area irradiated is small, a time period for treatment is long.
By the way, as a method for reducing dislocations caused in the nitride semiconductor thick film, an ELO growth (Epitaxial Lateral Overgrowth) method is proposed.
The conventional ELO growth method is exemplified in FIG. 19.
After an AlN buffer layer 321 is grown on a sapphire substrate 320 as shown in FIG. 19A, a first GaN layer 322 is grown as shown in FIG. 19B.
After that, a SiO2 film 325 is formed on the first GaN layer 322 as shown in FIG. 19C, and a slit line of SiO2 is formed in the [11-20] direction in mask treatment as shown in FIG. 19D.
Then, a arcsecond GaN layer 323 is grown again as shown in FIG. 19E.
Finally, the sapphire substrate 320 is separated as shown in FIG. 19F. The arcsecond GaN layer 323 grows from a slit window, and completely fills in the SiO2 line to become a flat film, because a speed of a longitudinal growth in the [1-100] direction is faster than a speed of growth in the [0001] direction. Although dislocation curves in a longitudinal direction in accordance with the longitudinal growth, and penetration dislocations on the SiO2 line can be reduced, the penetration dislocations concentrate on a part of the SiO2 slit window. Therefore, in order to produce a device by selecting a region with small penetration dislocation, it is only necessary to carry out mask treatment on the SiO2 line.
However, since SiO2 is filled in, it is difficult to carry out mask treatment on SiO2. Moreover, since curved dislocations concentrate on a central portion on the SiO2 line, there arises a problem of inclination of crystal orientation in a horizontal direction of the substrate, for example. Furthermore, since crystal growth is carried out while SiO2 is contained, diffusion of Si and oxygen atoms occurs. In addition, the ELO growth method needs a complicated production process, and therefore, brings about cost increase.
As described above, according to the conventional production methods, when the substrate made of a material of different kind from nitride semiconductor and the nitride semiconductor thick film are separated, stress resulting from differences in lattice constants and the coefficient of thermal expansion causes a warp and cracks on the produced nitride semiconductor apparatus. Moreover, the production process is complicated in the ELO growth that reduces dislocations, and it is difficult to keep away from a portion on the which penetration dislocation density concentrates, and to carry out the mask treatment on contained SiO2. Furthermore, there arises a problem of inclination of crystal orientation in a horizontal direction of the substrate because of curved dislocation, for example.